Development has progressed in recent years for electronic devices such as display devices, image sensors, or the like, equipped with optical sensor units utilizing TFDs.
Patent Documents 1 and 2 disclose image sensors that provide on the same substrate an optical sensor unit using a TFD and a drive circuit using a thin film transistor (TFT). According to Patent Document 1, due to crystallization of only part of an amorphous semiconductor film, from the same amorphous semiconductor film, an amorphous semiconductor layer is formed for producing the TFD active region and a crystalline semiconductor layer is formed for producing the TFT active region. Moreover, according to Patent Document 2, the TFT and TFD semiconductor layer is a crystalline semiconductor layer that is obtained by causing crystallization of the same amorphous semiconductor film.
Furthermore, Patent Document 3 proposes forming from the same amorphous semiconductor film TFD and TFT semiconductor layers that have mutually different crystallization states. Such a TFD and TFT, for example, can be used for a display device equipped with an optical type touch panel. Since this type of display device is equipped with an optical sensor using a TFD in the liquid crystal display device display region, this type of display device is able to utilize ambient light to sense an object such as a finger or the like contacting the display face.
FIG. 17 is a circuit diagram showing an example of structure of an optical sensor unit disclosed in Patent Document 3. The optical sensor unit has a thin film diode 1701 for an optical sensor, a capacitor 1702 for retaining a signal, and a thin film transistor 1703 for reading the signal accumulated in the capacitor 1702. After an RST signal is input and the RST potential is written to a node 1704, when the node 1704 potential drops due to photo leakage, the gate potential of the thin film transistor 1703 changes, and the TFT gate opens or closes. The signal VDD can be read out by this means.
On the other hand, in Patent Document 4, a display device provided with an optical type touch panel is proposed that uses a sensor for detection of invisible radiation to sense invisible radiation emitted from a backlight. According to the display device disclosed in Patent Document 4, radiation emitted from the backlight reflects from an object such as a finger or the like contacting the screen, and the reflected invisible light is detected by sensors arranged at each pixel of the display device.